I have a list of items, which I want to run an async task on each.
I want to synchronize so each element will be processed once the preceding element has completed. What I've tried so far is:
function processItems(items) {
var i = 0;
while(i < items.length) {
asyncFunc(items[i], function() { i++ }); // asyncFunc takes a callback parameter
}
}
However this loops forever (I believe i is out of scope in the callback function).
Is there a better way to achieve this?
I think the following accomplishes what you're looking for:
function processItems(items) {
var i = 0,
length = items.length,
fn = function() {
if(i < length) {
asyncFunc(items[i], fn);
i++;
}
};
fn();
}
fn is a function that sets the callback equal to itself as long as the index is less than the length. Then I call fn once to start it off. Here's a fiddle:
Alternatively, if asyncFunc returns a promise, you can use Array#reduce to process the items in a series:
function processItems(items) {
return items.reduce((promise, item) => {
return promise.then(() => asyncFunc(item));
}, Promise.resolve());
}
If you need to execute a set of async function in series, I highly recommend the async module for node.
It offers a series method to execute async tasks in series. It also offers a waterfall method that will pass the results of the previous task to the next task.
Oops. You are looking for eachSeries. Here's how it might work for you:
var async = require('async');
async.eachSeries(items, asyncFunc, function(err){ if(err) return "OH NO"; return "Yay!"})
Make sure asyncFunc invokes a callback with either an error or null.
